Cane toad tadpoles cannibalise their competitors

Cane toad tadpoles eating toad eggs (c) Mattias Hagman Cane toad tadpoles do not snack on their siblings

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Cane toad tadpoles cannibalise eggs to survive, say scientists.

The juvenile toads are known to eat each other, a process that starts when they are just a few days old.

Researchers have now found that the behaviour helps to reduce competition in addition to giving the cannibals a nutritional boost.

Cane toads are a threat to wildlife in Australia. These findings suggest that "persuading" them to eat each other could help control their population.

Start Quote

Toad tadpoles almost never encounter eggs that are closely related to them - so they can happily go ahead and munch any they find”

End Quote Professor Richard Shine University of Sydney

Researchers from the University of Sydney and James Cook University, Queensland in Australia, wanted to find out why cane toad tadpoles ate the eggs of their own species.

Their study, published in the journal Animal Behaviour, compared two groups of tadpoles of the same number: one was allowed to eat toad eggs and the other was prevented.

The team found that cannibal tadpoles survived, grew and metamorphosed into toads more successfully than the tadpoles that did not eat the eggs.

Competition reduction

Although the tadpoles benefited from the nutrition of the eggs, they also improved their chances for the future, according to Professor Richard Shine who lead the research.

"The most important benefit is not nutrition, but the reduction of competition from the tadpoles that otherwise would have hatched from those eggs," he said.

But the tadpoles' voracious appetites do not extend to their siblings, as Prof Shine explained.

Toad eggs (c) Michael Crossland Cane toad clutches contain thousands of eggs

"The tadpoles don't eat close kin eggs, because of the short incubation period and the long delay between successive clutches by a single female," he told BBC Nature.

"Thus, toad tadpoles almost never encounter eggs that are closely related to them - so they can happily go ahead and munch any they find, without the risk that they are eating their relatives."

Prof Shine's results build on his previous findings that cane toad tadpoles can detect eggs in a pond using their sense of smell.

"Toad tadpoles can use specific chemicals produced by toad eggs to locate those eggs and eat them," he explained.

"We were astonished to discover that these simple little creatures, with brains the size of a pinhead, can react in subtle ways to specific cues.

"The tadpoles have a secret chemical language that only they can detect and respond to."

Pest control

Cane toads are native to South America but were introduced to Australia in 1935 to control sugar cane pests.

CANE TOAD CANNIBAL FACTS

Juvenile toad eats another (c) Christa Beckmann
  • Up to 67% of a juvenile cane toad's diet can be fellow toads
  • They have adapted a special "toe-waving" technique to lure in others as prey
  • The toads have a habit of swallowing prey whole

They are now one of the International Union for the Conservation of Nature's (IUCN) top 100 invasive species and considered feral pests across north-eastern Australia.

Their large clutches of eggs and ability to migrate 40km per year are some of the primary reasons for their population explosion.

These scientists are studying the toads to find their biological weak spots. And Dr Shine's findings could help in efforts to control the toad populations and conserve native biodiversity.

"The high rates of targeted cannibalism mean that if we can encourage all of the local toads to spawn in the same pond, rather than in lots of different ponds, the toads will help to take care of the control problem for us by eating any newly-laid eggs."

Tadpole specialist Dr Richard Wassersug from Dalhousie University, Canada, said that exterminating "undesirable invasive species" would be easier "if you can get at them when they are locally concentrated".

However, he also warned that the species should not be underestimated.

"We need to be cautious about assuming that adult cane toads are oblivious to the presence of predators, such as older cannibalistic tadpoles, in ponds where they could lay their eggs," he commented.

In the past, other species of amphibian have been shown to make considered decisions about where to lay their eggs, relative to the risk of cannibalism.

Dr Wassersug added: "If adult cane toads have the same ability, then it will be much more difficult to get them to concentrate their egg-laying into one focal site."

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